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OANY

UNDERSTANDING OPTICAL PHENOMENA

By Linda Conlin, ABOC, NCLEC,FNAO

Release Date July 20th 2020

Expiration Date July 20th 2021

Course Description

This course provides an understanding of the visual pathway from the point where light enters the eye to interpretation of the image in the visual cortex. Optical illusions occur when interpretation of the images differs from objective reality. The course presents the types of physiological and cognitive optical phenomena and how they occur. You will also be able to experience optical phenomena including perceptual organization, depth and motion perception, color and brightness constancy and future perspective.

Objectives

At the conclusion of this course, attendees will have learned about:

1. The function of the visual pathway from the point where light enters the eye to interpretation of images at the visual cortex.

2. The types of optical phenomena: physiological resulting from the effects of excessive stimulation, and cognitive, resulting from the brain organizing stimuli into meaningful information.

3. How optical phenomena occur through identification and experience of examples of the types of phenomena in the physiological and cognitive categories.

Phenomenon vs. Illusion

A phenomenon is defined as an occurrence, circumstance, or fact that is perceptible by the senses. An illusion is an erroneous perception of reality. This course considers both that which the senses perceive as well as the brain’s interpretation.

Optical illusions aren’t new. Figure 1 is a mosaic found in a church dating back to 4 A.D. The boxes at the corners appear to be three dimensional, although the mosaic is flat.

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Figure 1

Is the hand quicker than the eye?

Not necessarily! The brain has to make sense of the sensory information it receives in microseconds, and so will anticipate incoming information based on what it has already received. Distraction disrupts the process, leading to erroneous conclusions.

The eye has optical and neural components. When those components are healthy, they relay images accurately. The optical components of the eye are the cornea and crystalline lens, which refract light to focus on the retina. The neural components of the eye include rods and cones that transmit images to the retinal ganglion cells. Chemical reactions in the ganglion cells stimulate the optic nerve.

From the optic nerve, neural impulses travel to the optic chiasm where half of the neural fibers cross to the opposite hemisphere. They end at the lateral geniculate nucleus (LGN) that gauges the range and speed of objects to anticipate movement. The LGN sends this information to the primary visual cortex of the brain. There are 6 distinct areas of the visual cortex that respond to different visual characteristics: V1-edge detection, spatial and color changes; V2-determines depth; V3-processes direction and speed; V4-recognizes simple shapes; V5-analyzes object motion; and V6-analyzes motion relative to background.

The eye provides; the brain decides

An optical illusion or phenomenon occurs when visual processing in the brain results in perception different from physical reality. The brain looks for known patterns in ambiguous information to make sense of the input.

Illusions can be physiological or cognitive. A physiological illusion is a physiological imbalance that alters perception. It can originate in the eye or brain. It is the result of the effects of excessive stimulation or competing stimuli of the same type.

Physiological illusion

One type of physiological illusion is afterimage. It is an image that persists after the visual stimulus that caused it ceases. Afterimages can be positive or negative. For a positive afterimage, with excessive stimulation, chemical activity in the retina continues to send neural impulses to the brain, so we continue to “see” the image even after it’s gone. For a negative afterimage, overstimulation of the photoreceptors in the retina reduces their sensitivity. The brain adapts to keep the image consistent. Adaptations continue to be processed for a short time after the stimulus ceases.

An example of a positive afterimage can be experienced using Figure 2. Stare at the white cross for 30 seconds, then look away at a white piece of paper. For a second or two, you should see the same image on the paper. (Note: Those with color vision deficiency may not see all of the images in these exercises.)

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Figure 2

An example of a negative afterimage can be experienced using figure 3. Stare at the cross on the bird’s wing for 30 seconds. Then look at the cross inside the birdcage. The bird will appear in the cage for a second or two, but with the colors of its breast and wings reversed.

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Figure 3

Lateral inhibition

Lateral inhibition is an optical phenomenon in which some retinal receptors receive more light than neighboring receptors, such as the white discs at the line intersections vs. the gray bars in Figure 4. This inhibits the firing of the nearby receptors, and so the white discs appear black. The effect transmits laterally.

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Figure 4

Simultaneous color contrast

Simultaneous color contrast is the phenomenon in which the perception of colors of objects seen together affect each other, changing our perception of the colors. The greatest effect is seen with complimentary colors. As cones for each color become fatigued, cones for the complimentary color activate, and we perceive the first color as having some hue from the complimentary color.

In Figure 5, which stripe matches the one in the box? The answer is A. The stripe in the box is a solid color, not shaded like stripe B. The complimentary red cones affect the perception from the fatigued green cones.

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Figure 5

Cognitive illusions

Cognitive illusions result from the brain organizing stimuli into meaningful information. The brain interprets the images based on many factors including memory, context and experience. Color, depth perception and relative size contribute to illusions.

Brightness constancy is one type of cognitive illusion in which the color of an object appears duller against a black field than against a white field. That is because black reflects less light than white, and retinal responses are in the context of the surroundings. When the brightness of the background is different from the object, an object will appear darker if the background is lighter than the object and appears lighter if the background is darker. Based on sensory evidence, perceived lighting conditions, and previous experiences, our visual system arrives at a false conclusion. You can see this effect in Figure 6. The two swatches are actually the same color.

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Figure 6

Color constancy

Color constancy is the tendency of objects to appear as the same color even under changing illumination. It is a perceptual phenomenon believed to aid with object recognition. For example, the brain recognizes that the strawberries in Figure 7 are red even through the blue filter and with all red pixels in the picture removed.

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Figure 7

Perceptual rivalry

Perceptual rivalry occurs when two images of equal strength are presented to both eyes simultaneously. Because we can be conscious of only one image at a time, one becomes dominant and the other is suppressed. The suppressed and dominant images will alternate due to neural fatigue or fluctuations in brain activity.

How many women are in Figure 8? The answer is one, but you may see two. You may also notice the image fluctuate between perceiving one and two women.

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Figure 8

Ambiguous image

An ambiguous image produces an effect similar to perceptual rivalry. Ambiguous images form two separate pictures from a single image. The effect is usually intentional by the artist, and as with perceptual rivalry, the pictures can alternate. In Figure 9, do you see a duck or a rabbit?

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Figure 9

Binocular rivalry

Binocular rivalry is also similar to perceptual rivalry, except that a different image is presented to each eye simultaneously. Because we can perceive only one image at a time, one image is dominant and one is suppressed. While the image the brain perceives as more “interesting” usually becomes the dominant image, the perceptual dominance will switch between the two images.

Delboeuf illusion

This illusion was documented by Belgian philosopher Franz Joseph Delboeuf in the 1860s. Delboeuf documented a perceived difference in the size of two identical circles when one is surrounded by a much larger circle, and another is surrounded by a circle that is only slightly larger. Studies have shown that this illusion can affect how much we eat. They've shown that people using smaller dishes overestimate the size of their servings, even as they serve themselves less food. Conversely, using larger dishes increases portion size. Considering that the average size of an American dinner plate has increased almost 23 percent since 1900, we may be overeating without realizing it. Figure 10 demonstrates this perception.

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Figure 10

Ponzo illusion

The Ponzo illusion is also about size perception relative to other visual cues. It is a geometrical-optical illusion that was first demonstrated by the Italian psychologist Mario Ponzo in 1911. He suggested that the human mind judges an object's size based on its background. The Ponzo illusion is often illustrated with two converging lines that mimic railroad tracks disappearing into the distance. Two horizontal lines or bars are placed across these "tracks," one higher than the other. When looking at the image, viewers routinely see the upper bar (where the converging lines are closer) as larger than the lower bar. In reality, the two bars are of identical size. The effect of the Ponzo illusion is often attributed to linear perspective. The upper line looks longer because we interpret the converging sides as parallel lines receding into the distance. In this context, we interpret the upper line as though it were farther away, so we see it as longer. In the three dimensional world, an object located farther away would have to be larger than a nearby object for both to produce retinal images of the same size. (Figure 11)

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Figure 11

Relative orientation – the tilt effect

With the tilt effect, the perceived orientation of a grating pattern is affected by surrounding grating of a different orientation. Orientations that differ from between 0 and 50 degrees appear to tilt opposite to the surrounding grating called repulsion effect. Orientations that differ up to 90 degrees appear to rotate toward the surrounding called attraction effect. The context of the surrounding grating alters the perception of the center grating. In Figure 12, the grating in all inner circles is at 90 degrees, but the tilt of the surrounding grating changes that perception.

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Figure 12

The Ames Room

Invented by ophthalmologist Adelbert Ames, Jr. in 1946, the room is a trapezoid designed to appear square. The brain prefers to change its perception of the person rather than the room. It is an example of how we judge size relative to familiar settings. The girl in Figure 13 is the same size and distance from the viewer, but the design of the room changes the perspective.

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Figure 13

Pulfrich effect

The Pulfrich effect is a psychophysical rule in which lateral motion of an object in the field of view is interpreted by the visual cortex as having a depth component, due to a relative difference in signal timings between the two eyes. Viewing an object moving side to side with a dark filter over one eye slows the visual signal slightly from that eye. The visual cortex interprets the difference in the signals as adding depth to the image. The back and forth path of a pendulum appears to become elliptical. The effect can occur spontaneously with eye diseases such as monocular cataract, optic neuropathy, color vision deficiency and lesions in the visual pathway. It is also seen without the use of a filter in patients with MS after optic neuritis in one eye slows optic nerve conduction. In these cases, the use of filters can reverse the neural delays and improve spatial perception. (Figure 14)

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Figure 14

Perception of motion

Differences in color shading stimulate the visual “motion detectors” in areas V5 and V6 of the visual cortex. This combined with natural saccadic eye movement results in the perception of motion. The same effect can be observed in black and white. (Figure 15)

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Figure 15



Zӧllner illusion

The Zöllner illusion was created in the mid-1800s by Johann Karl Friedrich Zöllner, a German astrophysicist. In this illusion, a central aspect of a simple line image appears distorted with respect to other aspects of the image. These are sometimes called geometrical-optical illusions. One explanation for this illusion is that our perceptual systems have a tendency to ‘expand’ acute angles—that is to represent them as larger angles than they really are, and, conversely, ‘contract’ obtuse angles. This causes one end of the longer lines in Figure 16 to seem closer than the other, even though all of the lines are perfectly parallel. Another explanation is that the illusion may be caused by an impression of depth. The fact that the shorter lines are on an angle to the longer lines may help to create the impression that one end of the longer lines is nearer to the viewer than the other end.

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Figure 16

Hering illusion

The Hering Illusion was created by Karl Ewald Konstantin Hering, a German physiologist, in 1861. Like the Zöllner illusion, it is a geometrical-optical illusion in which a central aspect of a simple line image appears distorted by other aspects of the image. In the Hering illusion, (Figure 17) the blue radial lines which intersect the vertical red lines cause the visual system to enhance the orientation contrast between the red and blue lines. This causes the appearance of the red lines bending outwards near the center, even though they are parallel. Another explanation is that it takes 100 milliseconds for an impulse from the retina to reach the brain. The brain learns to anticipate the image in “real time.” In the image, the blue lines give the visual impression of moving forward, so we would “see” the red lines getting further apart.

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Figure 17 By Fibonacci - Own work, CC BY-SA 3.0,

Paradox illusion

Paradox illusions are generated by objects that are paradoxical or impossible in "real life" or three dimensions, but look convincing in two dimensional drawings. Such illusions are often dependent on a cognitive misunderstanding that adjacent edges must join. These illusions are intentionally created by artists who purposely distort perspective. In trying to make sense of the visual input, the brain alters perception in favor of a concept. (Figure 18)

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Figure 18

Autostereogram

Similarly to the Hering and Paradox illusions, an Autostereogram creates the illusion of a 3D scene from a 2D scene. The method uses binocular disparity in a 2D repeating pattern to create the 3D effect. This happens because the brain perceives depth by comparing points of vision for each eye with the other. In rows of horizontally repeating patterns, images that are closer together appear to be closer to the viewer than the background. Images usually appear to converge in the distance. The wallpaper effect skews that convergence to a point further away than the image itself, resulting in a 3D effect. (Figure 19)

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Figure 19

Illusory contours

Illusory contours are based on the perception of objects as whole even when incomplete. The brain ignores gaps and we perceive complete contour lines to form familiar figures and shapes. The neurons in the brain that extract the illusory edges can identify only those edges defined by luminance differences because of the way in which neurons evolved. Individual neurons in areas 17 and 18 in the occipital lobe fire only when lines of a certain orientation are displayed in a specific location of what is called the “receptive field.” Many of them will respond only to a line of a specific length. If it is longer, they will stop firing. These are known as “end-stopped cells.” Neurophysiologist Rudiger von der Heydt of Johns Hopkins University suggested that these cells are signaling an implied occlusion that is effectively chopping off the line, and the cells then respond to illusory contours. Figure 20 actually has no triangles at all, only incomplete circles and lines.

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Figure 20

Vertical-horizontal illusion

The vertical–horizontal illusion is the tendency for observers to overestimate the length of a vertical line relative to a horizontal line of the same length. The bisecting line appears to be longer than the line that is bisected, possibly because we perceive the parts of the bisected line rather than the whole. Another factor in this illusion may be that humans seem to have a visual preference for vertical lines. (Figure 21)

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Figure 21

Troxler effect

The Troxler effect is named after Swiss physician and philosopher Ignaz Paul Vital Troxler. In 1804, Troxler made the discovery that rigidly fixating one’s gaze on some element in the visual field can cause surrounding stationary images to seem to slowly disappear or fade. They are replaced with an experience, the nature of which is determined by the background for the object. This is known as filling-in. Filling-in occurs when retinally fixed stimuli fade due to neural adaptation, meaning that neurons adapt to ignore unimportant stimuli.

The Troxler effect illustrates the importance of saccades, the involuntary movements of the eye which occur even while gaze is apparently fixed. If we could perfectly fixate on some point in our visual field by suppressing saccadic movement, a static scene would slowly fade from view after a few seconds due to the local neural adaptation of the rods, cones and ganglion cells in the retina. Any constant light stimulus will cause an individual neuron to become desensitized to that stimulus, and so reduce the strength of its signal to the brain. Saccadic movements don’t move the stimulus to a new receptor, and so the lack of variation in the stimulus is ignored.

If you stare at Figure 22 for a few seconds, it will begin to disappear.

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Figure 22 ; Thomson, G. and Macpherson, F. (July 2107), "Troxler Effect" in F. Macpherson (ed.), The Illusions Index. Retrieved from .

Conclusion

Optical phenomena can be physiological or cognitive, with cognitive as the most common. Physiological phenomena depend largely on the state of the neural receptors. In cognitive phenomena, the eye provides, the brain decides. A healthy visual system will send accurate images to the brain, but the brain interprets the images based on many factors including memory, context and experience. Color, depth perception and relative size contribute to illusions.

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